Image forming device with a built-in cleaning blade, so far known in the art, include one having such construction as shown typically in FIG. 1. The image forming device is built up of a photoconductive drum 1, a cleaning unit 2, a charging unit 3, an exposure unit 4, a developing unit 5, a transfer unit 6, and so on. The photoconductive drum 1 is rotated in a direction indicated by an arrow A. The surface of the photoconductive drum 1 is uniformly and consistently charged by the charging unit 3. An image exposure at the exposure unit 4 causes charges on an exposed area to vanish, so that an electrostatic latent image is formed on the photoconductive drum 1.
The electrostatic latent image on the photoconductive drum 1 is developed by a developer fed from the developing unit 5 (which is called an “electrostatic image developing toner” or referred to simply as a “toner”) to form a visible (toner) image. The developing unit 5 includes a developing roll 8 and a toner layer thickness control member 9, whereby stored toner is fed to the surface of the photoconductive drum 1. The toner image on the surface of the photoconductive drum 1 is transferred onto a transfer material 7 such as transfer paper via the transfer unit, and then sent to a fixing unit (not shown).
The photoconductive drum 1 is an image bearing member for carrying electrostatic latent images and toner images. A toner image on the photoconductive drum 1 is transferred onto the transfer member 7 via the transfer unit 6; however, a part of the toner often remains untransferred. Such untransferred toner on the photoconductive drum 1 should be removed by the cleaning unit 2 or else images will smudge upon image formation.
In the image forming device depicted in FIG. 1, the cleaning unit 2 includes a cleaning blade 2a and a support member 2b, which are located around the photoconductive drum 1. The cleaning blade 2a is positioned in contact at its end portion with the surface of the photoconductive drum 1. FIG. 2 is illustrative in perspective of the cleaning blade 2a and support member 2b. The cleaning blade 2a is usually bonded to the support member 2b by means of an adhesive.
The cleaning blade is a sheet-form member made of an elastic material such as rubber or polyurethane. For efficient removal of untransferred toner on the image bearing member, the cleaning blade is brought into contact at its end portion (edge) with the surface of the image bearing member at a suitable pressure. However, the cleaning blade made of an elastic material has an increased surface friction resistance although having an improved elasticity; if there is no equilibrium between the pressure applied on the image bearing member and frictional force, there will often be the so-called “curling-up” phenomenon, in which the end portion of the cleaning blade is dragged and bent out in the rotational direction of the image bearing member.
To avoid the “curling-up” phenomenon, it has hitherto been proposed to coat the edge of a cleaning blade with a lubricant comprising fine particles having a particle diameter of up to 20 μm and an improved flowability, and various proposals have been made as to how to coat powdery lubricants for the purpose of enhancing the adhesion thereof to the surface of the cleaning blade.
For instance, Japanese Patent No. 3112362 proposes a method that comprises applying a lubricant-containing solution, wherein a solid lubricant is dispersed in an aqueous solution with an acrylic resin emulsified or suspended therein, to at least a part of the portion of a cleaning blade to come into pressure contact with an image bearing member, followed by drying. In one specific example, the publication shows that a cleaning blade with an edge having a friction coefficient of 0.4 is obtained by applying a lubricant-containing aqueous dispersion with a film thickness of 6 μm (corresponding to the amount of attached powders of 0.6 mg/cm2) in the vicinity of the edge of a polyurethane cleaning blade, said aqueous dispersion being obtained by dispersion of polytetra-fluoroetheylene powders in an acrylic water emulsion.
Japanese Patent Application Laid-Open No. 7-266463 discloses a method of washing a cleaning blade prepared by bonding an elastic material blade to a support member with an aliphatic hydrocarbon detergent, and coating under pressure fluororesin fine powders on the cleaning blade before the detergent is dried up. With this method, the lubricant could be applied to the end portion of the cleaning blade to decrease frictional force without recourse to solvents such as flon, which are detrimental to environmental health.
Japanese Patent Application Laid-Open No. 8-220962 proposes a method of coating a portion of a cleaning blade to come in pressure contact with an image bearing member with a lubricant dispersion in which a powdery lubricant is dispersed in a fluorine inert liquid having a specific range of surface tension, followed by drying. The publication teaches that the powdery lubricant is preferably configured to a spherical shape. In one specific example, the publication shows that a cleaning blade coated and surface-treated with a powdery lubricant in an amount of 0.75 mg/cm2 is obtainable by dropwise addition and coating of a lubricant dispersion onto the edge portion of a polyurethane blade, followed by drying, said lubricant dispersion being obtained by dispersion of a spherical polymethyl methacrylate having an average particle diameter of 0.5 μm in a fluorine inert liquid (C6F14).
According to such surface treatment methods that use powdery lubricants, a suitable degree of lubrication could given to the end portion of the cleaning blade to come into contact with an image bearing member, so that friction with the surface of the image bearing member could be reduced thereby avoiding the “curling-up” phenomenon. The cleaning blade surface-treated with the powdery lubricant could keep good cleaning capability over a relatively long period of time, when applied to an image forming device using pulverized toner as a developer. However, it is now found that a cleaning blade surface treated with a conventional powdery lubricant is poor in cleaning capability when applied to an image forming device using spherical toner obtained as by suspension polymerization.
A toner production method is generally divided roughly into a pulverization process and a polymerization process. In the pulverization process, a binder resin and a colorant are hot milled together with other optional additive or additives, and the milled product is pulverized and classified into toner (pulverized toner). The pulverized toner is so made of aspheric particles having a broad particle size distribution that it can be easily removed by a cleaning blade. Problems with the pulverized toner are, however, that classification yields are low because of a considerable amount of fine powders generated at the pulverization step, and that the binder resin, because of being fragile, is finely divided during use, resulting in poor image quality.
By contrast, the polymerized toner is obtainable in the form of spherical colored polymer particles having a sharp particle diameter distribution by dispersing a minute droplet form of a monomer composition containing, for instance, a polymerizable monomer, a colorant and other optional additive or additives in an aqueous dispersion medium for suspension polymerization. Specifically, by control of polymerization conditions, the polymerized toner may be obtained in the form of substantially spherical colored polymer particles that have a sharp particle diameter distribution of 1.0 to 1.4 as expressed in terms of the dv/dn ratio where dv is a volume average particle diameter and dn is a number average particle diameter and a sphericity of 1.0 to 1.3 as expressed in terms of the dl/ds ratio where dl is a particle's major axis and ds is a particle's miner axis. For instance, see Japanese Patent Application Laid-Open No. 5-188637 and WO00/13063.
To meet recent demands for higher-definition images, higher printing speeds and full-color images, for instance, toners are now required to have (1) a smaller volume average particle diameter of up to 10 μm, preferably up to 9 μm, and more preferably up to 8 μm, (2) a sharper particle diameter distribution and a higher degree of sphericity, and (3) a lower fixation temperature without detrimental to shelf stability.
To suit such requirements, polymerized toners having ever smaller particle diameters and ever sharper particle diameter distributions are now under development. To achieve spherical toners having low-temperature fixing ability and shelf stability in a well-balanced state, a capsule toner having the core-shell structure, for instance, is developed by a two-stage polymerization process, by which a colored polymer particle having a low glass transition temperature is surrounded by a polymer layer having a high glass transition temperature.
With the development of toners having a smaller particle diameter, a sharper particle diameter distribution and a higher degree of sphericity simultaneously with improvements in low-temperature fixing ability, shelf stability and durability, however, it is more and more difficult to clean off toner particles remaining unfixed on an image bearing member. Spherical toner particles having a sharp particle diameter distribution are by far much larger than aspheric toner particles having a broad particle diameter distribution in terms of adhesive forces among toner particles and between toner and an image bearing member. In addition, such adhesive forces become strong with a decreasing toner particle diameter. For an image forming method that relies on spherical toner having smaller particle diameters, it is now required to have an improved method for removal of toner particles remaining untransferred on an image bearing member by use of a cleaning unit with a built-in cleaning blade.
With a cleaning method using a cleaning blade made of an elastic material, it is difficult to provide satisfactory removal of spherical, small-diameter toner particles remaining untransferred on an image bearing member. With a conventional cleaning blade wherein a relatively small amount of a powdery lubricant is attached at its end portion having good flowability (e.g., that set forth in Japanese Patent No. 3112362 or Japanese Patent Application Laid-Open No. 8-220962), it is prima facie possible to avoid the “curling-up” phenomenon by virtue of its decreased friction coefficient; however, it is still difficult to achieve satisfactory removal of spherical, small-diameter unfixed toner particles, and even images developed on only a limited number of sheets smudge due to poor cleaning capability.
With a prior method that involves washing a cleaning blade with an aliphatic hydrocarbon detergent and applying under pressure fluororesin fine powders thereon (e.g., that set forth in Japanese Patent Application Laid-Open No. 7-266463), it is inherently difficult to control the amount of the attached fluororesin fine powders because the aliphatic hydrocarbon detergent is likely to volatilize off, resulting in poor reproducibility. Indeed, Japanese Patent Application Laid-Open No. 7-266463 makes no specific reference to the amount of the attached fluororesin fine powders.
Improvements in the cleaning capability of spherical, small-diameter toner may be achieved by increasing the amount of silica or other abrasive particles to be added to the toner; however, this may adversely affect developability and transferability. Improvements in cleaning capability by configuring toner particles to shapes other than a spherical shape are not preferable because not only are such particles difficult to produce but also they render developability and transferability worse.